I need to waterproof a wooden sash window in the bathroom, to protect
it from the shower.

It's already had a coat of oil paint, but from past experience that
won't be enough in the long term.

I was thinking of applying 3 coats of Rustins Yacht Varnish. This does
have the disadvantage of gradually discolouring over time, but would I
imagine otherwise do the trick.

Does anyone have any comments on my plan, or suggestions for
alternatives (I'm not interested in changing the window, or resiting
the shower)?

Thanks for your help,

Chris